1. Field of the Invention
The present invention relates to a method for forming electrodes and, more particularly, to a method for forming an electrode on an electronic part made of ceramic such as chip parts, ceramic capacitors and active filters.
2. Description of the Prior Art
In the field of electronics, a higher degree of reliability of soldering technique, is strongly required through miniaturization of electronic parts, higher integration of printed circuits, finer pitching, automatization and higher speed of loading. Migration of silver used for external electrodes is likely to occur during soldering and there is now adopted a method of forming a nickel layer as barrier on the external electrodes for prevention of migration of silver. Also, covering the tin layer or solder layer by electrolytic plating, is adopted for prevention of bonding failure due to poor wettability of solder.
Also, a method of activating the electrodes by dipping it in a noble metal solution such as of palladium before electroless nickel plating, is usually used. Further, there is disclosed in Laid-open Application Publication No. 52827/1984 a method for covering and activating the electrodes by means of electrolessly plated palladium.
Also, for formation of a solder layer or a tin layer on a nickel plated surface by an electrolytic method, a method of activating the nickel plated surface by acid scouring immediately before the electrolytic plating is known.
However, since in the conventional method of forming a nickel layer, solder layer or tin layer by electrolytic plating, no plated layer can be formed unless under conditions allowing continuity, there is no degree of freedom with regard to the size and shape of the electronic parts to which the method is applicable. Furthermore, when there exist open terminals or short terminals in combination as a result of multiplication or multiple polarization of electronic parts, there results a marked variation of plating layer thickness. This tendency is marked especially with multi-polar network parts in which resistance (R), inductance (L) and capacitor (C) are conjugated.
Therefore, it is necessary to determine the electrolytically plating conditions itemwise, and this results in an increased manufacturing cost. In the future, the electrode interval is supposed to be narrower and the electrode size is supposed to be smaller, and these result in increased difficulty of determination of plating conditions.
Also, in order to establish electrodewise continuity in electrolytic plating such as nickel plating, tin plating and soldering, steel balls are put into the plating bath as media. When the steel balls and the electronic parts to be plated are stirred in the plating bath by means of a barrel and the like, minute cracks or chipping may result to affect the part's properties. Further, with multipolar parts, electrode's size is bound to decrease with decreasing part's size. Therefore, it is necessary to use still smaller steel balls in order to accomplish unfailing plating of such electrodes. However, when the steel balls used are too small compared with the electronic parts, the electronic parts are likely to flow up in the electrolytic plating bath off the steel balls, and this results in deterioration of the quality of the deposited layer. Also, this results in reduction and resulting deterioration of the electronic parts caused by the active hydrogen generated in the course of electrolytic plating.
Though there is disclosed in Laid-open Application Publication 52827/1984 tin plating or soldering by electrolytic or electroless plating, nothing is disclosed about concrete examples. Furthermore, there is disclosed in Laid-open Application Publication 52827/1984 electroless soldering bath using 3-valence titanium ion as reducing agent. But, when solder layer is formed on electroless nickel layer by said method with this plating bath, precipitation speed is slow and it is difficult to obtain solder layer having enough wettability of solder.